The present invention relates to the field of cryogenic air distillation using an air separation unit (xe2x80x9cASUxe2x80x9d) comprising more than one cryogenic distillation column. The present invention has particular application to an ASU having a thermally integrated double column distillation system comprising a higher pressure (xe2x80x9cHPxe2x80x9d) column and a lower pressure (xe2x80x9cLPxe2x80x9d) column.
The distillation columns of an ASU have a plurality of column sections. The hydraulic loading of the various column sections can vary significantly and it is common to use two or more different diameters for the column sections, especially when structured packing is used as the mass transfer elements in the columns.
The upper sections of the LP column of a double column system usually determine the largest diameter used in the column system, as it is at this location that typically the column system has the largest volumetric flow of vapor. For a defined maximum column diameter in the double column system, the upper sections of the LP column are usually the bottleneck for the capacity rating of the column system. The HP column and lower sections of the LP column would allow a higher plant capacity if their diameters were increased towards the stated maximum diameter value. If the double column capacity could be increased without increasing the maximum double column section diameter then the footprint of the column system and associated piping would be largely unchanged.
An advantage of reducing the flow bottleneck in the upper sections of the LP column would be that the capacity of the double column system could be increased (under the constraint of a particular defined maximum column diameter). In addition, the ability for very large columns to be shipped is often determined by the maximum column section diameter. If the above flow bottleneck could be reduced then the maximum capacity of a single train double column could be increased.
U.S. Pat. No. 5,100,448 (published on Mar. 31, 1992) discloses a column system using structured packing, where a lower density (higher capacity) structured packing is used in column sections having a high hydraulic load and higher density (lower capacity) packing is used in sections having a low hydraulic load. While this could achieve the objective mentioned above, low density packing has substantially poorer mass transfer performance than higher density packing.
U.S. Pat. No. 6,128,921 (published on Oct. 10, 2000) discloses an arrangement of multiple LP columns to increase the capacity of the plant, with each LP column providing part of the product. It does not address the problem that it is only the upper sections of the LP column that cause the initial capacity bottleneck for the double column system.
U.S. Pat. No. 6,227,005 (published on May 8, 2001), WO-A-84/04957 (published on Dec. 20, 1984) and an article in Research Disclosure by Richard Mason Publications entitled xe2x80x9cIntermediate Pressure Column in Air Separationxe2x80x9d (No. 425, September 1999, pp 1185 to 1186, XP-000889172) all disclose processes for the production of oxygen and nitrogen using a distillation column system having at least three distillation columns, each column operating at a different pressure and each intermediate pressure column having at least one reboiler/condenser.
The purpose of the intermediate pressure column in both U.S. Pat. No. 6,227,005 and WO-A-84/04957 is to pre-separate a liquid oxygen containing feed stream into further enriched bottoms liquid and oxygen lean overhead gas.
It is an object of the invention to provide an ASU comprising a multiple column distillation system having an increased capacity within the constraint of a defined maximum column section diameter. The inventor has found that this can be achieved by routing a small fraction of the vapor flow which would normally pass through the upper LP column sections through an auxiliary separation column which is refluxed by a liquid stream from or derived from the HP column. Usually, the vapor flow rate in the auxiliary column is less than about 25%, preferably less than about 20% and most preferably less than about 15%, of the vapor flow rate in the upper LP column sections. Bottoms liquid from the auxiliary column is returned to the LP column at an intermediate location above the bottom section.
Air is separated cryogenically using a multiple column distillation system comprising at least an HP column and an LP column. The process comprises feeding cooled feed air to the HP column for separation into HP nitrogen-enriched overhead vapor and crude liquid oxygen (xe2x80x9cCLOXxe2x80x9d) and feeding at least one LP column feed stream comprising nitrogen and oxygen to the LP column for separation into LP nitrogen-rich overhead vapor and liquid oxygen (xe2x80x9cLOXxe2x80x9d), the LP column being refluxed with a liquid stream from or derived from the HP column. Oxygen-containing gas comprising no more than about 50 mol % oxygen is fed to an auxiliary separation column for separation into auxiliary column nitrogen-rich overhead vapor and oxygen-rich liquid. Oxygen-rich liquid from the auxiliary column is fed to an intermediate location in the LP column and the auxiliary column is refluxed with a liquid stream from or derived from the HP column.
In preferred embodiments, the purpose of the auxiliary column is to unload the upper sections of the LP column by feeding it with a gas stream that would otherwise have had to pass through the LP column upper sections.
The apparatus comprises an HP column for separating cooled feed air into HP nitrogen-enriched overhead vapor and CLOX, an LP column for separating at least one LP column feed stream comprising nitrogen and oxygen into LP nitrogen-rich overhead vapor and LOX, conduit means for feeding a liquid stream from or derived from the HP column as reflux to the LP column; an auxiliary separation column for separating oxygen-containing gas comprising no more than about 50 mol % oxygen into auxiliary column nitrogen-rich overhead vapor and oxygen-rich liquid, conduit means for feeding oxygen-rich liquid from the auxiliary column to an intermediate location in the LP column and conduit means for feeding a liquid stream from or derived from the HP column as reflux to the auxiliary column.